1. Field of the Invention
The invention is concerned with the art of applying coatings to parts, more particularly to the art of applying coatings to elongate parts, such as metal strips. The invention has particular application to applying hardenable metallic coatings onto strip seal members although it will be appreciated that the invention is not limited thereto but is generally applicable to coating a plurality of parts which are capable of being assembled to form two oppositely disposed coating surfaces with the portions (faces) to be coated recessed with respect to the portions (backs) which are not ultimately to be coated. Generally, the method of the invention is applicable to elongate members having oppositely facing fronts and backs, and sides which are configured so that the parts may be assembled and held in side-by-side abutting relationship, with some part faces facing in one direction and some in the opposite direction, the parts being staggered if necessary to have the faces recessed relative to the backs.
The invention has particular applicability to the application of hardenable metallic coatings to metal strip seals such as the application of hard facing coatings to so-called side or compression seals used in rotary piston ("Wankel") internal combustion engines.
2. Description of the Prior Art
It is often desirable or necessary to apply coatings to parts, such as hard or wear-resistant coatings to metal parts to provide a wear-resistant bearing or sealing surface thereon.
Known methods of applying hard facing coatings to Wankel engine side seals provide an example of prior art methods. Such coating has heretofore been carried out by assembling a plurality of side seals, which comprise elongated arcuate metal strips, into a stack of strips. This is conveniently done by clamping strips within a clamping fixture or jig so that the faces of the strips define a coating surface onto which a suitable hard facing coating may be applied. A plasma gun or flame spraying apparatus may conveniently be used to spray a hardenable metallic coating on the strip faces. It may be that the faces to be coated contain grooves or other recesses which are to be filled with the coating while the unrecessed portions of the faces are to be free of coating. In such case, a continuous coating is applied over the surface defined by the assembled faces. The hardened coating is thereafter ground while the parts are held in place within the fixture, to reduce the thickness of the coating sufficiently to expose the unrecessed portion of the faces. This leaves a residual coating within the grooves. However, if the face of the strip is to be coated, from edge to edge, or if unrecessed edge portions are to be coated, a difficulty arises in attempting to separate the coated pieces. This is so because the coating, bridging one piece to the next piece in an unbroken layer, tends to adhere the pieces together.
One prior art expedient for overcoming this difficulty is exemplified by U.S. Pat. No. 3,337,938, and provides recessed shims or spacers between the parts to be coated so that the parts are spaced one from the other and the coating does not bridge adjacent parts. The coating is directed in a small controlled spray to avoid bridging the parts to the shims. Another prior art expedient is to utilize spacers made of berylium copper or other material which does not accept the metal coating. It will be appreciated that the use of such spacers is expensive and inconvenient, particularly since the spacers cannot be normally be reused.
Another example of a prior art expedient is set forth in U.S. Pat. No. 3,850,669, wherein cylindrical parts are assembled with cylindrical spacer plates therebetween. Both parts and spacer plates are uniformly coated. Thereafter, the coating is cracked along a predetermined line of weakness between the parts and the spacers. This involves added manufacturing steps and waste of coating material.